Furnance running, blowing warm air, turns off when heated to correct temp but doesn't come back on again. Go to thermostat to turn up temp, does not activate system to turn on. Can switch from Auto to ON and system will run, not really doing anything but circulating air. When turned to ON, outside system does nothing.

Have had issue twice in three days. When this issue first happened over the weekend, I noticed top of outside unit was caked in iced snow. Removed snow, spun the fan, and when I adjusted temp on thermostat, the system kicked on like no issues.

We had the system inspected and cleaned in the summer, good results after for A/C. System is at least 20yrs old.

I read possible capacitor or fan motor but anything spot on thoughts? How hard to change capacitor and/or fan motor? How can I track down parts?

sounds like it could be a fan motor or capacitor issue outside, but without more info, that could be wrong. have you been able to observe a defrost cycle in its entirety ? if there is ice buildup outside, periodically, the defrost cycle will turn off the outside fan motor, but continue to run the compressor. additionally, the reversing valve will de-activate and run hot refrigerant gas through the outside coil to melt off ice (ice buildup is normal during heat mode below 32 degrees). once ice is melted off, the system returns to normal heat operation and the outdoor fan will run again.

in normal heat mode, the larger of the two freon lines will be hot. this is how you will know it is not in defrost. if it is not in defrost, the fan should be turning. if the heat pump is malfunctioning, the aux heat will kick in and maintain temp inside.

run the following test -

turn your thermostat off until the house is 55 degrees. in heat mode, raise the thermostat setpoint to 70 degrees. observe the outside unit the entire time it's running. is the larger of the two freon lines hot ? is the fan running outside ? is there ice on the coil ? does a defrost cycle occur within the first hour ? what is the temperature outside ? if the defrost cycle occurs, the fan should turn off (outside) and compressor should keep running. when this happens, does the larger of the two freon lines change to cool ? how long does this cycle last ? is the ice gone ? when it returns to normal mode, the fan should start running again and the larger line should go hot again. is this the case ?

after the above test, set the thermostat to desired setpoint. after it cycles off, time the off cycle. also note the temperature drop at the thermostat. if it does not cycle back on after the temp drops 4 degrees, note the time lapsed. report these findings for further response. thanks.

I found similar solutions yesterday and started running thru the steps. (To begin in the heating mode, start at the thermostat. Ensure that it is on heat and set above the room temperature. Now, turn the fan to on. If the fan doesn't start, then check the fuses/breakers for the air handler or furnace. If the breaker was tripped, the problem could be a bad wiring connection, the blower motor, or the control board. If the breaker was not tripped, the problem could be the thermostat, low voltage wiring, or the fan relay. Now that the fan is running, turn the thermostat to emergency heat. Ensure that the setpoint is at least 5 degrees above room temp. Wait for about a minute, then check to see if warm air is coming out the vents. If there is, then the problem is with the outdoor unit. If there is not, the problem could be the thermostat or something in the air handler.)

Our problem is that when we go to turn up the heat on the thermostat, it's not accepting the override temp. We had a alt thermostat laying around and switched the two just to see if faulty t-stat but same result. Furnance will only run in ON mode, doesn't heat. In the instructions above, it list to turn to Emergency Heat and since I'm not sure we have that opt, I've got to pull the t-stat manuals.

my impression from your original post was that your fan was not really the issue. you said it would run and heat, then cycle off. the fan override would run if you turned it on (fan on/auto switch in on position)). when you raise the setpoint, you got no response inside or outside. your fan running with the override switch rules out a breaker or fuse problem, but does not rule out a control problem. since you tried the other thermostat with same results, it's not likely the thermostat.

for clarification, have you been able to get the system to run since your original post ? have you been able to observe all the functions outside ?